Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 248,931	$ 181,385
Interest on Convertible Debt, Net of Tax	0	56
Net Income (Loss) Available to Common Stockholders, Diluted	$ 248,931	$ 181,441
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Weighted average shares outstanding - basic	105,572	104,290
Effect of dilutive securities (in shares):
Stock options	9,050	8,147
Restricted stock	484	469
Incremental Common Shares Attributable to Dilutive Effect of Conversion of Debt Securities	0	44
Warrants	0	1,278
Dilutive potential shares	9,534	9,938
Weighted average shares - diluted (in shares)	115,106	114,228
Earnings Per Share, Basic and Diluted [Abstract]
Net income per share - basic	$ 2.36	$ 1.74
Net income per share - diluted (in dollars per share)	$ 2.16	$ 1.59
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of shares	11,535	7,539
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of shares	18	19